1. Field of the Invention
The present invention relates to a scraper filter system incluing scraper means, and more particularly to a filter system in which an improved scraper means consisting of a plurality of separate scraper segments is provided and proper scraping operation is steadily maintained.
2. Description of the Prior Art
In the prior art scraper filters disclosed in Japanese patent applications, such as the application nos. 1-107834 and 1-223302 by the same applicant as in the present invention, a scraper made of a wear resistant material is provided at the edge of a spiral impeller in a cylindrical filter, constantly engaged with the inner area of the cylindrical filter, and a filtrate from a feed liquid such as highly viscous liquid food is discharged from the outside of the cylindrical filter while the feed liquid is delivered with no loss of feeding pressure from the inlet to the outlet port for the feed liquid until the concentrated food solid is finally taken out from the cylinder. The filters have an advantage that since the scraper is prepared in the ways that coil springs are incorporated into the scraper made of a wear resistant long plastic board and fixed in a groove formed along the spiral edge of the impeller, the edge of the scraper is closely engaged with the inner area of the cylindrical filter in virtue of expansion of the coil springs.
On the other hand, the aforementioned prior art filters with scrapers have the following problems.
When the pressure of the scraper to the filter becomes high due to either extremely narrow slits or holes on the filter or excessively high viscosity of a feed liquid, abrasion on the edge of the scraper much progresses, and the length of the spiral scraper is reduced at large, which makes the scraper shifted along its groove by the reduced length toward rotating direction of the spiral impeller, resulting in occurrance of the scraper's not engageing to the filter at the feed liquid inlet in the cylindrical filter.
The phenomenon described above is mathematically explained as follows.
When the diameter of the cylindrical filter is R(cm) and a worn length on the scraper is .alpha.(mm), the 1 pitch length of the scraper worn by .alpha.(mm) and engages the filter surface can be expressed: EQU (R-0..multidot..alpha.).pi.(cm) (1)
(the length is considered as a circle)
On the other hand, since the 1 pitch length of the scraper with no wear and engage with the filter surface is regarded as the same as the inner circumference of the filter, the 1 pitch length can be expressed: EQU R.pi.(cm) (2)
(the length is considered as a circle)
Since the reduced length in the 1 pitch length of the scraper worn by .alpha.(mm) and contacted to the filter surface is the length difference between (2) and (1), the reduced length in the 1 pitch length of the scraper can be expressed: EQU R.pi.(cm)-(R-0.multidot..alpha.).pi.(cm)=0.multidot..alpha..multidot..pi.(c m) (3)
When the number of the pitch in the whole spiral impeller is n, the absolutely reduced length of the scraper can be expressed: EQU 0.multidot..alpha..multidot..pi..times.n(cm) (4)
And when .alpha. is 2(mm) and n is 20, the absolutely reduced length of the scraper becomes from formula(4): EQU 0.2.times.3.14.times.20(cm)=12.56(about 13)(cm)
As mentioned above, there is the problem in our previously devised filters with scrapers which were filled as Japanese patents (the application Nos. 1-107834 and 1-223302). That is as the wear of the scraper increases, the length of the scraper becomes shorter, the scraper is shifted by the reduced length toward the rotating direction of the spiral impeller, which brings about reduction of scraping efficiency in the way that between the scraper and the filter some unengaged area occurs.